Single photon emission (SPE) imaging is a known medical imaging technique. It involves injecting radiopharmaceutical substance into a patient's body and evaluating the distribution of the radiopharmaceutical substance, which is indicated by the distribution of gamma rays emitted from within the patient's body.
A radiation-detecting system, often referred to as Gamma camera, detects those gamma rays. Gamma camera detects gamma rays emitted from the radiopharmaceuticals substances, and the data acquired is analyzed to form an image representing the distribution of concentrations of the radiopharmaceutical substance within a specific body area.
Several modalities of SPE imaging are in use:
One of them is Single Photon Emission Computerized Tomography (SPECT). In this technique the gamma camera rotates around the region of interest of the patient's body, and data are collected at several angular positions (hereafter referred to as angular projections). A fully three dimensional image is formed.
SPECT is considered to be a very useful technique and a good tool for obtaining diagnostic information, however it requires the collection of large number of emitted photon (large statistics) and this means that in order to obtain the required number of photons, a long acquisition time is necessary. Long acquisition time means that the patient is subjected to a relatively long period of discomfort and furthermore, making the overall number of patients who can be imaged in a given time relatively small—a feature that many medical institutes and hospitals regard as extremely unfavorable and undesirable.
In order to reduce acquisition time planar or static imaging technique is sometimes used.
Unlike SPECT, in planar imaging techniques, the detector is kept at a fixed angular position in relation to the patient's body. The formed image is essentially a two-dimensional single projection of the administered radiopharmaceutical substance concentration within the patients body and lacks the three-dimensional information provided by SPECT (more than one projection may be acquired, but each projection is treated separately, yielding a single image).
The popularity of static nuclear imaging stems from the shorter time it takes to acquire an image, possibly reduced radiation exposure and reduced discomfort to the patient. Shorter imaging time enables scanning of large portions, or even the full length, of the patient's body, by either obtaining several images, possibly partially overlapping, or by slowly moving the camera along the patient body. Sometimes, several consecutive images of the same portion of the patient's body are taken in order to allows the physician to follow the dynamic redistribution of radiopharmaceutical substance in it, and assess the functioning of organs.
In prior art planar imaging, the image is in fact a representation of the distribution of incident photons across the gamma camera detector, without any additional image processing or with general image processing techniques, for example smoothing, which are pure mathematical manipulations of the collected data regardless of the physical nature of the system. In other words, if a planar single photon emission image undergoes any image processing, this processing does not take into account any physical parameters associated with the system (the nature of radiopharmaceutical substances and the patient's anatomy).
Gamma camera generally comprises a photon detector crystal coupled with a plurality of photomultiplier tubes, or an array of solid-state detectors combined with position logic circuits and data analysis apparatus. A collimator for limiting the angle of incident gamma rays may be incorporated with the gamma camera. Collimators are used to limit the detection of photons to a predetermined range of incidence angles (photons with greater incidence angles are absorbed by the collimator septa). A collimator typically includes thousands of squares, round or hexagonal parallel channels, through which, and only through which, gamma rays are allowed to travel and reach the detector. Generally, a parallel-hole collimator is in use, however various other arrangements may be used.
As gamma rays are emitted from the radiopharmaceutical substance, they travel through the collimator, unabsorbed and interact with the detector, which is placed directly adjacent to the collimator. The interactions of the gamma rays with the detector crystal create flashes of light in a process called scintillation. The scintillation light is preferably detected by an array of photomultiplier tubes, which are normally coupled to the back of the crystal. Photomultiplier tubes are used when a very small amount of light is emitted in scintillation. The output signals from the photomultiplier tubes are electric pulses, proportional to the energy of the gamma rays. The electric pulse output is received by position logic circuits, which determine the position where the scintillation event had occurred on the detector. Similarly, in solid-state detectors including solid-state crystals, the incident photons produce electric current corresponding to the energy of the incident photon in the specific location of incidence. This current is picked up by electrodes coupled to the solid-state crystals and is processed. The data is processed by position logic circuits and is transferred to a processing computer in order to process the data into readable image of the spatial distribution of the radiopharmaceutical substances within the patient's body. The main limitations to the quality of SPE images comes from:
A) Geometric resolution of the collimator. Getting information about the location where a photon was emitted requires limiting the incidence angle of those accepted for detection within predetermined ranges. The narrower the range, the better the resolution, but also the fewer the number of collected photons. Resolution is thus limited by sensitivity, and vice versa. High-resolution collimators typically used in the art reject photons at angles larger than 2 degrees, while high-sensitivity collimators reject those at angles larger than 3 degrees. The camera spatial resolution depends on the geometric resolution of the collimator, and degrades with distance between the surface of the collimator and the organ being imaged.
B) Limited number of registered gamma ray photons. a) The constraints imposed on the sensitivity of the collimator by geometrical resolution needs (typically only 1, out of every 10,000 emitted photons, is collected), b) the low doses of radiopharmaceutical that can be administered to patients due to hazards associated to exposure to gamma radiation, c) limited acquisition time.
All three factors limit the number of registered photons. This together with the random nature of radioactive decay turns the data into only a statistical (noisy) representation of the actual activity within the body of the patient.
Gamma ray photons, particularly of high energy, have some probability of penetrating the collimator septa, and reaching the detector even though their angle of incidence exceeds the collimator acceptance angle.
Gamma ray detectors have finite resolution. The ability of the position logic circuits in a scintillation camera to determine the precise position of a scintillation event is primarily determined by the number of photons generated in the crystal by this event, and thus the detector resolution strongly depends on the gamma ray energy. In multi-crystal detectors, resolution mainly depends on the size of the individual crystals.
Compton scattering within the patient, in the collimator or in the detector may cause a gamma ray photon to be registered in an erroneous place.
U.S. Pat. No. 4,873,632 (Logan, et al), titled APPARATUS AND METHODS FOR SCATTER REDUCTION IN RADIATION IMAGING, filed in 1990, discloses discrimination of counted photons based on measured energy, using the energy information for correction of scattering and smoothing the resulted image to reduce noise. Iterative reconstruction methods are used for SPECT. U.S. Pat. No. 6,943,355 to the same assignee discloses a method for image reconstruction that results in enhanced three-dimensional nuclear image.
It is an object of the present invention to provide an image (hereinafter also referred to as—planar image) representing two-dimensional projection of the three-dimensional distribution of radiopharmaceutical substance administered to a patient.
It is another object of the present invention to provide a method for enhancing planar nuclear images.
It is an object of the present invention to provide a gamma camera adapted to enhance planar single photon emission imaging.
It is another object of the present invention to provide a gamma camera adapted to provide a shorter acquisition time and thus relatively reducing the discomfort of a patient.
Yet another object of the present invention is to provide a gamma camera having improved image sensitivity and resolution.
Still another object of the present invention is to provide a gamma camera having improved lesion detectability.
Furthermore, it is another object of the present invention to provide a method for enhancing the quality of planar single photon emission imaging.
Furthermore, it is another object of the present invention to provide planar single photon emission imaging obtained from relatively smaller amount of radiopharmaceutical substances.
Furthermore, it is another object of the present invention to provide planar single photon emission imaging having reduced image-processing time.
More objects and advantages of the present invention will become apparent from the following detailed descriptions when read in conjunction with the accompanying drawings.